.\"	$OpenBSD: SSL_CTX_set_alpn_select_cb.3,v 1.7 2018/03/23 14:28:16 schwarze Exp $
.\"	OpenSSL 87b81496 Apr 19 12:38:27 2017 -0400
.\"	OpenSSL b97fdb57 Nov 11 09:33:09 2016 +0100
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.\" This file was written by Todd Short <tshort@akamai.com>.
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.Dd $Mdocdate: March 23 2018 $
.Dt SSL_CTX_SET_ALPN_SELECT_CB 3
.Os
.Sh NAME
.Nm SSL_CTX_set_alpn_protos ,
.Nm SSL_set_alpn_protos ,
.Nm SSL_CTX_set_alpn_select_cb ,
.Nm SSL_select_next_proto ,
.Nm SSL_get0_alpn_selected
.Nd handle application layer protocol negotiation (ALPN)
.Sh SYNOPSIS
.In openssl/ssl.h
.Ft int
.Fo SSL_CTX_set_alpn_protos
.Fa "SSL_CTX *ctx"
.Fa "const unsigned char *protos"
.Fa "unsigned int protos_len"
.Fc
.Ft int
.Fo SSL_set_alpn_protos
.Fa "SSL *ssl"
.Fa "const unsigned char *protos"
.Fa "unsigned int protos_len"
.Fc
.Ft void
.Fo SSL_CTX_set_alpn_select_cb
.Fa "SSL_CTX *ctx"
.Fa "int (*cb)(SSL *ssl, const unsigned char **out,\
 unsigned char *outlen, const unsigned char *in,\
 unsigned int inlen, void *arg)"
.Fa "void *arg"
.Fc
.Ft int
.Fo SSL_select_next_proto
.Fa "unsigned char **out"
.Fa "unsigned char *outlen"
.Fa "const unsigned char *server"
.Fa "unsigned int server_len"
.Fa "const unsigned char *client"
.Fa "unsigned int client_len"
.Fc
.Ft void
.Fo SSL_get0_alpn_selected
.Fa "const SSL *ssl"
.Fa "const unsigned char **data"
.Fa "unsigned int *len"
.Fc
.Sh DESCRIPTION
.Fn SSL_CTX_set_alpn_protos
and
.Fn SSL_set_alpn_protos
are used by the client to set the list of protocols available to be
negotiated.
The
.Fa protos
must be in protocol-list format, described below.
The length of
.Fa protos
is specified in
.Fa protos_len .
.Pp
.Fn SSL_CTX_set_alpn_select_cb
sets the application callback
.Fa cb
used by a server to select which protocol to use for the incoming
connection.
When
.Fa cb
is
.Dv NULL ,
ALPN is not used.
The
.Fa arg
value is a pointer which is passed to the application callback.
.Pp
.Fa cb
is the application defined callback.
The
.Fa in ,
.Fa inlen
parameters are a vector in protocol-list format.
The value of the
.Fa out ,
.Fa outlen
vector should be set to the value of a single protocol selected from the
.Fa in ,
.Fa inlen
vector.
The
.Fa out
buffer may point directly into
.Fa in ,
or to a buffer that outlives the handshake.
The
.Fa arg
parameter is the pointer set via
.Fn SSL_CTX_set_alpn_select_cb .
.Pp
.Fn SSL_select_next_proto
is a helper function used to select protocols.
It implements the standard protocol selection.
It is expected that this function is called from the application
callback
.Fa cb .
The protocol data in
.Fa server ,
.Fa server_len
and
.Fa client ,
.Fa client_len
must be in the protocol-list format described below.
The first item in the
.Fa server ,
.Fa server_len
list that matches an item in the
.Fa client ,
.Fa client_len
list is selected, and returned in
.Fa out ,
.Fa outlen .
The
.Fa out
value will point into either
.Fa server
or
.Fa client ,
so it should be copied immediately.
If no match is found, the first item in
.Fa client ,
.Fa client_len
is returned in
.Fa out ,
.Fa outlen .
.Pp
.Fn SSL_get0_alpn_selected
returns a pointer to the selected protocol in
.Fa data
with length
.Fa len .
It is not NUL-terminated.
.Fa data
is set to
.Dv NULL
and
.Fa len
is set to 0 if no protocol has been selected.
.Fa data
must not be freed.
.Pp
The protocol-lists must be in wire-format, which is defined as a vector
of non-empty, 8-bit length-prefixed byte strings.
The length-prefix byte is not included in the length.
Each string is limited to 255 bytes.
A byte-string length of 0 is invalid.
A truncated byte-string is invalid.
The length of the vector is not in the vector itself, but in a separate
variable.
.Pp
For example:
.Bd -literal
unsigned char vector[] = {
	6, 's', 'p', 'd', 'y', '/', '1',
	8, 'h', 't', 't', 'p', '/', '1', '.', '1'
};
unsigned int length = sizeof(vector);
.Ed
.Pp
The ALPN callback is executed after the servername callback; as that
servername callback may update the SSL_CTX, and subsequently, the ALPN
callback.
.Pp
If there is no ALPN proposed in the ClientHello, the ALPN callback is
not invoked.
.Sh RETURN VALUES
.Fn SSL_CTX_set_alpn_protos
and
.Fn SSL_set_alpn_protos
return 0 on success or non-zero on failure.
WARNING: these functions reverse the return value convention.
.Pp
.Fn SSL_select_next_proto
returns one of the following:
.Bl -tag -width Ds
.It OPENSSL_NPN_NEGOTIATED
A match was found and is returned in
.Fa out ,
.Fa outlen .
.It OPENSSL_NPN_NO_OVERLAP
No match was found.
The first item in
.Fa client ,
.Fa client_len
is returned in
.Fa out ,
.Fa outlen .
.El
.Pp
The ALPN select callback
.Fa cb
must return one of the following:
.Bl -tag -width Ds
.It SSL_TLSEXT_ERR_OK
ALPN protocol selected.
.It SSL_TLSEXT_ERR_NOACK
ALPN protocol not selected.
.El
.Sh SEE ALSO
.Xr ssl 3 ,
.Xr SSL_CTX_set_tlsext_servername_arg 3 ,
.Xr SSL_CTX_set_tlsext_servername_callback 3
.Sh HISTORY
.Fn SSL_select_next_proto
first appeared in OpenSSL 1.0.1 and has been available since
.Ox 5.3 .
.Pp
.Fn SSL_CTX_set_alpn_protos ,
.Fn SSL_set_alpn_protos ,
.Fn SSL_CTX_set_alpn_select_cb ,
and
.Fn SSL_get0_alpn_selected
first appeared in OpenSSL 1.0.2 and have been available since
.Ox 5.7 .
